The invention pertains generally to composite propellants and in particular to burning rate modifiers for ammonium perchlorate composite propellants.
Ferrocenes are used in solid rocket propellants as effective burning rate catalysts for ammonium perchlorate (AP)-based systems. Ferrocene and its derivatives are very rapidly oxidized in the burning propellant to iron oxide particles of submicron size which act as catalytic sites to accelerate the gas-phase decomposition of perchloric acid (formed from decomposing AP). In addition to rate enhancement, ferrocene compounds are useful in reducing the dependence of burning rate on pressure for fast burning propellants.
The development and use of ferrocene and its derivatives in solid propellants have presented several problems. Ferrocene is a rather high-melting solid that sublimes at moderate temperatures. It cannot be used in propellants because of its tendency to migrate through the propellant grain and crystallize at the surface. Use of liquid alkyl ferrocenes, such as those disclosed in A. T. Nielson et al. J. Org. Chem. 41 p. 655-9 1976 has eliminated the crystallization problem, allowed better dispersion of iron throughout the propellant, and provided better processing qualities by acting as a plasticizer. Unfortunately, most of the alkyl ferrocenes are sufficiently volatile for significant quantities to be lost during processing and for sensitivity hazards to arise due to contact with ammonium perchlorate dust in the mixer. These additives migrate badly in the cured propellant and are easily oxidized because of their alpha-hydrogen structure. Ease of oxidation and migration are responsible for increased sensitivity and erratic burning in propellants.
Large, nonvolatile, nonreactive multi-ferrocene molecules, such as those disclosed in Carl Gotzmer et al. Non-Migrating Ferrocene Modifiers for Composite Propellants. 1979 JANNAF Propulsion Meeting, Vol. II, PP 475-83, March 1979, were developed to overcome migration and processing problems. These were thought to be less likely to migrate through the propellant grain because of greater chain entanglement with the binder. Aging studies, however, indicated that migration and oxidation problems still existed but to a lesser degree. For example, aging data for a series of CTBN propellants prepared using one weight percent of bis (2-ferrocenylethyl) disulfide, which is retained by the propellant binder polymer via chain entanglement and polar forces, indicated that plasticizers played a major role in the ferrocene compound's migratory aptitude and that its use should be restricted to propellants containing no plasticizer and to storage conditions below 60.degree. C.(140.degree. F.).
Recent efforts have centered on chemically bonding ferrocene derivatives to the propellant matrix. Such a procedure would ensure that the burning rate modifier remains uniformly distributed throughout the propellant while retaining its catalytic effectiveness. Aging qualities of the propellant would thereby be much improved over those where accelerators are not chemically bonded. Various approaches have been used to react ferrocene additives with certain components of a propellant formulation.
Binders have been prepared which contain ferrocene as an integral part of the polymer. For example, copolymers of vinyl ferrocene and butadiene, disclosed in U.S. Pat. No. 3,886,190 by S. F. Reed, issued on May, 1975, have been used as binders for ammonium perchlorate-based propellant systems and have produced burning rate increases of about 20 percent. Other examples of ferrocene-polymers used as propellant binders are disclosed in U.S. Pat. No. 4,168,362 by Gotzmer et al and U.S. Pat. No. 3,886,007 by Combs, Jr. et al. Difficulties, however, have been encountered in obtaining high ferrocene-content polymers that retain good mechanical properties.
Incorporation of ferrocene into the binder matrix via the curing agent also ensures nonmigration and complete dispersion of the modifier. For example, ferrocene derivatives containing hydroxyl and/or isocyanate functional groups, can be joined to a HTPB binder network via crosslinking binder network via crosslinking and chain extending. Migration of ferrocenes tied to the binder matrix by this method has been proven to be non-existent. However, the quantity of ferrocene incorporated into the propellant formulation is limited by the stoichiometry of the crosslinking system.
Coating ammonium perchlorate with certain tertiary-amine derivatives, e. g. aziridinylmethyl ferrocene (AMF) of H. M. Fisher Multipurpose Additives for Composite Propellants. RK-TR-69-6. PP 10-12, May, 1969 (declassified on 1972), 1-pyrrolidinylmethyl ferrocene (PMF) of O. E. Ayers et al. Multipurpose Additives for Composite Propellants, Part II. RR-TR-70-8. PP 1-5 March, 1970, (declassified on 1973), and N, N-dimethylaminomethylferrocene (DAMF) of C. Gotzmer et al., ibid, decreases migration. The main disadvantages of AMF are that (1) the aziridinyl group can interfere with the propellant binder cure reaction (therefore, quantities that can be used are limited), and (2) AMF-coated AP is extremely impact, friction, and thermally sensitive when dry. The sensitivity and safety problems of AMF are also found with the other two ferrocenes.